"With the off-days, we have a chance with Doug coming back to insert him into the rotation," Williams said. "Given that likelihood, we felt it important to add an extra bullpen guy, and work out that rotation as we go."

Fister suffered a lat strain late in Spring Training and has been on the disabled list all season. While Williams said he hopes to keep the same order in the rotation, that's contingent on Fister feeling healthy in his final rehab start on Friday.

"Obviously, we need to see how [Doug] does in his last rehab start, and have him feel good about it for this timetable to stick," Williams said. "If that happens, we're looking at [May] 7th, maybe the 8th. He's close."

Chris Abshire is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.